A radionavigation receiver measures observables (e.g., pseudorange and Doppler shift) from RF signals which are used to compute an estimate of position, velocity and time (PVT). Because these observables are a function of antenna motion, if the radionavigation receiver embedded in a smartwatch or other wearable computer is worn on a wrist of a swimmer the PVT estimate can be inaccurate due to the inclusion of the velocity of the wrist in the PVT estimate. Additionally, because the radionavigation receiver can receive RF signals only when above water, the PVT estimate may be inaccurate due to the radionavigation receiver being periodically submerged under water.